The Gram-negative bacterium Acinetobacter baumannii is a leading cause of nosocomial infections in humans. The emergence of A. baumannii strains resistant to most, and in some cases, all available antibiotics has made the treatment of these infections exceedingly difficult. This underlies the importance of finding new ways to combat these infections. This proposal addresses regulation of a high-frequency phenotypic switch in A. baumannii that controls virulence. The goals of this proposal are to understand regulatory mechanism responsible for this switch and identify/characterize a quorum sensing signal that stimulates this switch.